Festive Cider and Garlic Mussels

Ingredients

• 1.5kg mussels
• A knob of butter
• 1-2 tablespoons rapeseed, olive or sunflower oil
• A couple of garlic cloves, bashed or chopped
• 1 large or 2 medium leeks, finely sliced
• An apple, cored and diced
• 1 teaspoon thyme leaves (optional)
• 1 bay leaf (optional)
• 250ml cider (or apple juice, or both)
• 1 teaspoon mustard
• 2 tablespoons double cream (optional)
• freshly ground black pepper
• Lots of chopped parsley, to serve